An optical head, whether of the bulk or integrated optic variety, must provide at least three separate functions. First of all, it must retrieve the data signal encoded on the light reflected off of the optical disk. Second, it must measure tracking errors, which are displacement errors associated with motion of the disk in the plane of the disk. Finally, it must measure focus errors, which are displacement errors associated with motion of the disk in a direction perpendicular to the plane of the disk. Integrated Guided Wave Optical Heads (IGWOH) represent compact, low mass alternatives to bulk optical head assemblies for reading information from an optical data storage disks.
Current integrated optical head designs such as the one proposed by Ura et al. in An Integrated-Optic Disk Pickup Device, Journal of Lightwave Technology, Vol. LT-4, No. 7, July 1986 employ conventional techniques such as the "push-pull" method for monitoring the tracking error signal (TES) and the "pupil obscuration" method for monitoring the focus error signal (FES). (See, for example, "Principles of Optical Disk Systems", G. Bouwhuis et al., Adam Hilger Ltd., Boston, 1985.) These methods are adapted to optical waveguides using grating lenses and beam splitters to replace the bulk optical components. Unfortunately, these methods require high performance lenses (i.e., lenses with large numerical apertures and large Strehl ratios) which are extremely difficult to fabricate in waveguides. Furthermore, the fabrication techniques required for these lenses are not readily adaptable to mass production.
U.S. Pat. No. 4,798,437 discloses a method and apparatus for processing analog optical wave signals using Mach-Zehnder (MZ) interferometer arrays formed on inorganic electro-optic substrates, such as LiNbO.sub.3 for example, to analyze wavefront profiles. Among the devices mentioned is an integrated optical waveguide range finder wherein free space radiation from a point source is end-fire coupled into an array of channel waveguides. The distance of the point source from the analyzer determines the curvature of the wavefront incident on the analyzer. Portions of the wavefront falling on adjacent channel waveguides are shifted slightly in phase due to this curvature. Adjacent channels are joined together at a Y-junction and the relative phase shift results in interference between light from these two channels. The signal exiting the interferometer, when taken together with the signals from other interferometers in the array, provide a measure of the wavefront curvature and hence of the distance to the source. Other light signals provide intensity reference levels. Control electrodes permit electro-optical phase-shifting of one channel with respect to the other. This feature provides a means for compensating for slight differences in the optical path lengths of the two channels or of biasing one channel with respect to the other.
Even though channel waveguide MZ interferometry has been used to sense wavefront curvature, it has not been used to measure the focus error signal in an integrated optic head. Further, prior art interferometer arrays are based upon waveguides formed by indiffusion in LiNbO3 or other similar inorganic electro-optic single crystals. Accordingly, it will be appreciated that it would be highly desirable to use newer technology and have waveguides formed from new thin-film electro-optic polymers deposited on silicon substrates and to integrate electronic functions such as photodetection, amplification, and switching with the optic functions.